Preheat the oven to 325 degrees F. Line 2 baking sheets with parchment paper or baking mats.
Cream the butter and sugar together in an electric mixer fitted with the paddle attachment until blended, for approximately 30 seconds. Sift the flour and cornstarch together in a bowl, and then add to the mixer and mix until the ingredients all come together. Finally, sprinkle over the black pepper and mix again. Tip the dough out onto a board and form it into 2 discs. Wrap each in plastic wrap and refrigerate 20 minutes.
Place a piece of parchment paper on the counter, approximately 12-by-15-inch in size, and dust with flour. When the dough has rested, unwrap the first ball, put on top of the parchment paper, flour the surface of the dough and rolling pin and roll out to a 9-inch diameter circle, 1/16-inch thick.
Trim the circle into an 8-by-8-inch square, discarding the scraps. Then cut down in 2-inch strips and across in 2-inch strips, making sixteen 2-by-2-inch squares. Using a small offset spatula, place the squares on the prepared baking sheet. Sprinkle a pinch of flaked sea salt over each cracker. Repeat the process with the second dough ball.
Bake for 18 minutes. Do not allow the crackers to brown; check your oven sooner if it runs hot. Set aside to cool for 20 to 25 minutes before packaging up.